What is a common mode inductor

 

Common mode inductor, also known as common mode choke, is often used to filter common mode electromagnetic interference signals in computer switching power supply. In the board design, the common mode inductor also plays the role of EMI filtering, which is used to suppress the outward radiation and emission of electromagnetic waves generated by high-speed signal lines.

 

Theoretical knowledge

 

The main board inside the computer is mixed with various high-frequency circuits, digital circuits and analog circuits. When they work, they will produce a large number of high-frequency electromagnetic waves to interfere with each other, which is EMI. EMI will also be emitted through mainboard wiring or external cables, causing electromagnetic radiation pollution and affecting the normal operation of other electronic equipment.

 

The chip on PC board is not only an electromagnetic interference object, but also an electromagnetic interference source. In general, we can divide these electromagnetic interferences into two categories: serial mode interference (differential mode interference) and common mode interference (grounding interference). Taking the two PCB wires on the main board (wires connecting the components of the main board) as an example, the so-called serial mode interference refers to the interference between the two wires; The common mode interference is the interference caused by the potential difference between the two wiring lines and the PCB ground wire. The series mode interference current acts between two signal lines, and its conduction direction is consistent with the waveform and signal current; The common mode interference current acts between the signal line and the ground wire. The interference current flows through one-half of the two signal lines in the same direction, and the ground wire is the common loop.

 

If the common mode current generated by the board is not attenuated and filtered (especially the common mode current on the high-speed interface line such as USB and IEEE 1394 interface), the common mode interference current can easily generate electromagnetic radiation through the interface data line - the common mode radiation generated by the common mode current in the cable. The FCC of the United States, cispr22 of the international radio interference special committee and GB9254 of China all have relevant restrictions on the common mode conducted interference and radiation emission of the communication port of information technology equipment. In order to eliminate the input interference signal and induced interference on the signal line, we must reasonably arrange the filter circuit to filter the interference of common mode and series mode. Common mode inductance is an integral part of the filter circuit.

 

The common mode inductor is essentially a bidirectional filter: on the one hand, it should filter out the common mode electromagnetic interference on the signal line, on the other hand, it should inhibit itself from emitting electromagnetic interference, so as to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment

 

Working principle of common mode inductor

 

Why can common mode inductors prevent EMI? To understand this, we need to start with the structure of common mode inductor.

 

The filter circuit of common mode inductance, La and LB are common mode inductance coils. The two coils are wound on the same iron core with the same number of turns and phase (winding reverse). In this way, when the normal current in the circuit flows through the common mode inductor, the current generates a reverse magnetic field in the inductor coil wound in the same phase and offsets each other. At this time, the normal signal current is mainly affected by the coil resistance (and a small amount of damping caused by leakage inductance); When a common mode current flows through the coil, due to the isotropy of the common mode current, a magnetic field in the same direction will be generated in the coil, which will increase the inductive reactance of the coil, make the coil show high impedance and produce strong damping effect, so as to attenuate the common mode current and achieve the purpose of filtering.

 

In fact, if one end of the filter circuit is connected to the interference source and the other end is connected to the interfered equipment, La, C1, LB and C2 form two groups of low-pass filters, which can control the common mode EMI signal on the line at a very low level. The circuit can not only suppress the incoming of external EMI signal, but also attenuate the EMI signal generated by the line itself, which can effectively reduce the intensity of EMI interference.

 

A small common mode inductor produced in China adopts high-frequency noise suppression strategy and common mode choke coil structure. It has the advantages of no signal attenuation, small volume, convenient use, good balance, convenient use and high quality. Widely used in double balanced tuning devices, multi frequency transformers, impedance transformers, balanced and unbalanced conversion transformers Wait.

 

There is also a common mode filter inductor / EMI filter inductor, which adopts ferrite core, double wire parallel winding, good noise suppression countermeasures, high common mode noise suppression and low differential mode noise signal suppression, low differential mode noise signal suppression interference source, difficult to deform in high-speed signal, small volume, good balance, convenient use, high quality and so on. Widely used in suppressing EMI noise of electronic equipment, USB line of personal computer and peripheral equipment, IEEE1394 line of DVC and STB, LCD panel, low voltage differential signal Wait.

 

Well, that's all for today's sharing. If you have any questions, please leave a message in the comment area. See you next time.